1. Field of the Invention
The present invention relates to a zoom lens system having a vari-focal function, in addition to the ability to zoom from a wide position to a tele position, that enables the present invention to set, at a position exceeding the tele position and discontinuous therefrom, at least one focal length or a lateral magnification longer than the focal length or higher than the lateral magnification at the tele position, respectively, so that photography is permitted at the focal length or the lateral magnification at the position exceeding the tele position.
2.Description of the Prior Art
In the recent year where cameras are fully automated, it is generally practised to widen photographing ranges by adopting zoom lens systems in the so-called lens shutter cameras which are portable in spite of the multiple functions imparted thereto In order to widen photographing ranges, it is necessary to develop a compact zoom lens system which can be built as a photographing optical system into camera systems.
Further, such a photographing optical system can have a short back focal length since it is unnecessary to reserve a space for arranging a quick return mirror in the photographing optical system unlike the optical systems to be used in the so-called single reflex cameras.
On the other hand, shortening total length of the photographing optical system over the entire zooming range from the wide position to the tele position thereof is conflicting in designing with enhancement of varifocal ratio thereof.
Zoom lens systems have high vari-focal ratios in the recent trend and there are known numerous compact zoom lens systems each of which has a high vari-focal ratio and consists of two lens units, i.e., a front lens unit having positive refractive power and a rear lens unit having negative refractive power. The zoom lens systems of this type have simple compositions and are used widely in practice. However, the zoom lens systems have a common defect that they cannot have large aperture ratios, though vari-focal ratios of the lens systems can be enhanced to an order of 2.
In order to correct this defect, there have been developed, as zoom lens systems having high varifocal ratios and high performance, the zoom lens systems disclosed by Japanese Patent Kokai Publication No. 63-43115 and Japanese Patent Kokai Publication No. 63-153511. However, it cannot be said that these zoom lens system have sufficiently high vari-focal ratios.
Furthermore, in order to further widen varifocal ranges, there have been developed the zoom lens systems which are adapted so as to permit photographing at focal lengths other than the focal lengths within the normal continuous vari-focal ranges. Most of these zoom lens systems are designed so as to permit arranging an additional vari-focal lens unit on the image side of a main lens system (a zoom lens system) or an afocal conversion lens on the object side of the main lens system.
These zoom lens systems are not preferable for practical use from the viewpoints of compactness of cameras and integral design with cameras.
As is understood from the foregoing description, the conventional zoom lens systems for compact cameras are insufficient in the vari-focal ratios thereof and inadequate from viewpoints of practical use such as integral design with cameras.